Phosphatidic acid phosphatase (PAP) catalyzes the dephosphorylation of phosphatidic acid (PA) to yield diacylglycerol (DAG) and Pi. The DAG generated in the reaction is used for the synthesis of triacylglycerol (TAG), and for the synthesis of phosphatidylethanolamine and phosphatidylcholine via the Kennedy pathway. By the nature of its reaction, PAP also controls the cellular content of PA, which is the precursor of phospholipids synthesized via the CDP-DAG pathway. PA is also a signaling molecule that triggers phospholipid synthesis gene expression, membrane expansion, vesicular trafficking, secretion, and endocytosis. Biochemical and genetic studies to establish the roles of PAP in lipid metabolism became possible by our discoveries of the PAP-encoding genes from the yeast Saccharomyces cerevisiae (e.g., PAH1) and mammals (e.g., LPIN1). The importance PAP in lipid metabolism is exemplified by its mutant phenotypes. In yeast, pah1 mutants exhibit defects in the transcriptional regulation of phospholipid synthesis genes, the anomalous expansion of the nuclear/ER membrane, and a 90 % reduction in TAG content in stationary phase cells. Studies with mice and humans have shown that genetic defects in lipin 1 and lipin 2 are manifested in several metabolic diseases that include lipodystrophy, obesity, peripheral neuropathy, myoglobinuria, and inflammation. In the next grant period, we will focus on the regulated expression of the yeast PAH1-encoded PAP enzyme during growth and its role in lipid metabolism (specific aim 1). In addition, we will expand our studies on PAP to include the enzymology and biochemical regulation of the human LPIN1-, LPIN2-, and LPIN3-encoded PAP isoforms (specific aim 2). We also discovered a novel CTP-dependent DGK1-encoded DAG kinase in yeast that counterbalanced the activity of PAP to control the cellular contents of PA and DAG. The dgk1 mutation suppresses phenotypes caused by the pah1 mutation, whereas the overexpression of DAG kinase blunts phenotypes caused by the overexpression of PAP. In yeast, TAG hydrolysis (e.g., lipolysis) to DAG and fatty acids is a prerequisite for membrane phospholipid synthesis and resumption of growth from the stationary phase. We will examine the role and regulation of the DGK1-encoded DAG kinase enzyme in this metabolism (specific aim 3).